Technology description of Thermal spray and HVOF
Hard chrome
alternatives summary – thermal sprays (primary alternatives
in blue).
|
Alternative |
Compliance |
Usage |
Notes |
|
HVOF carbides
|
RoHS, WEEE, ELV, OSHA Cr* |
Industrial hydraulics, rolls, airframes and engines |
Primary hard chrome alternative |
|
HVOF alloys (e.g. Tribaloys, stainless steels)
|
RoHS, WEEE, ELV, OSHA Cr* |
Hydraulic IDs, bearing journals, airframes and
engines, |
Less wear resistant than carbides, but less brittle
and more machinable |
|
HVOF NiAl |
RoHS, WEEE, ELV |
Worn components |
Relatively soft, used for thick build-up. HVOF
equivalent to sulfamate Ni |
|
Plasma and arc spray carbides and alloys
|
RoHS, WEEE, ELV, OSHA Cr* |
Worn components, journals, etc, turbine engine
components, hydroelectric turbines |
Being replaced by HVOF for aircraft but adequate for
most other industrial use. Major use in engines is
thermal barriers, not Cr replacement |
|
Cold (kinetic) spray materials
|
RoHS, WEEE, ELV, OSHA Cr |
Still in development |
Most only capable of spraying soft materials and
alloys. Carbide sprays not commercially proven |
|
*Note: There could be OSHA Cr
PEL issues (none yet seen) with some coating
materials if sprayed outside enclosed booths |
The most
commonly used technology for replacing hard chrome plating
is thermal spray (primarily, but not exclusively, HVOF).
While this is the technology of choice for most applications
it is not necessarily the best choice for all.
Technology
Thermal spray is the general term for a
number of processes in which particles of coating material
are heated and sprayed using high speed gas onto the surface
to be coated. It is a fully commercial industrial process
that is used for coating everything from bridges and radar
towers to aircraft parts and prosthetic knee joints. Since
thermal spray coatings are readily sprayed up to 0.020” (500
microns) thick, they are good for rebuilding worn
components, which is the primary use for hard chrome plate.
-
Of the various thermal spray technologies
(click here for comparison), the most widely used are
-
HVOF for wear resistant
coatings,
-
Zn and ZnAl for corrosion
coating of metal structures such as communication towers
and bridges, and
-
Plasma
spray for thermal barrier coatings and abradables in gas
turbines.
Major thermal spray processes.
|
Process |
Coating material |
Heat source |
Usage |
|
HVOF (High Velocity Oxy-Fuel) |
Powder (tungsten and chrome
carbide, Ni alloys) |
Supersonic flame (oxy-hydrogen,
natural gas-oxygen, kerosene) |
Aircraft components, hydraulic
actuators for mining and heavy equipment, industrial rollers –
primary wear coating alternative to chrome plate. Most
expensive, highest quality. Most commonly used to replace
hard chrome |
|
Plasma spray |
Powder (tungsten and chrome
carbide, Ni alloys, oxides) |
Intense plasma |
Wear coatings, thermal barriers.
Usually not as hard and dense as HVOF but some new types of
spray are comparable |
|
Arc spray |
Wire |
Electric arc |
Primary use is to spray Zn and Al
for corrosion protection of bridges, communications towers,
concrete |
|
Flame spray |
Alloy powder |
Subsonic flame |
Repair of some components, Zn and
Al coatings for corrosion. Least expensive, lowest quality |
|
Cold spray (kinetic spray) |
Alloy powder |
Hot or cold gas |
High velocity spray gives particles
enough velocity to form coating with little or no added heat.
In development and early marketing |
Availability
There are a large number of companies around
the US (most major cities have one or more companies) and
elsewhere in the world who supply thermal spray services,
and the equipment and supplies (thermal spray powders) are
available from a number of international companies.
In the US there are at least a dozen
aerospace-qualified thermal spray vendors and many who serve
the industrial segment.
Use of HVOF on aircraft
HVOF WC-CoCr is taking
over from hard chrome plate on most new commercial aircraft
made by Boeing and Airbus. It is now used on landing gear,
hydraulic actuators, flap and slat tracks, and other wear
surfaces. In addition, airlines are increasingly using HVOF
coatings to replace hard chrome in maintenance operations,
both because of its better performance and faster
maintenance turnaround.
-
Landing gear of Boeing
767-400
-
Landing gear of Airbus
380
-
Landing gear of Boeing
787
-
Landing gear and
actuators of military aircraft such as the F-35 and X-45
UCAV.
Use of HVOF hydraulic actuators
Although hard chrome
plate is still by far the most common wear coating used on
hydraulics, such as those used in industrial equipment, HVOF
coatings are being used increasingly by manufacturers and
repair centers, partly to avoid the problems associated with
chrome plating and partly for performance.
-
Heavy equipment
-
Trucks
-
Mining equipment
-
Elevators
Use of HVOF on industrial rolls
HVOF coatings are widely
used on industrial rolls, especially for repair. They can
be finished to a very smooth surface that is highly wear
resistant. Rolls that are often coated include mill rolls,
calendar rolls, anilox rolls, conveyer and conductor rolls.
Advantages
HVOF is a widely
available technology that has been in production use for a
number of years and is fully aerospace-qualified, with many
commercial and OEM specifications. In general it has a
number of advantages over chrome plate:
-
No Cr6+ – Even
when the spray powder contains Cr metal (WC-CoCr) or
chromium carbide ceramic (Cr3C2-NiCr),
the material is not in hexavalent form. The most common
chrome alternative powder used in aircraft is WC-CoCr.
The HVOF process is not known to emit Cr6+;
the only place that Cr6+ is known to be
formed in production thermal spray processes is in
plasma spray of Cr2O3, which is
used for Anilox print rolls.
-
Relatively simple
industrial process – Commercially available equipment
and materials.
-
Able to build up from
0.001” to >0.020” – same range as hard chrome.
-
Much better wear
resistance than hard chrome – Typically 3x or more
life. Tends not to scratch or damage on assembly and
disassembly. Avoids striations and scratches on
hydraulic rods.
-
Better
corrosion resistance – Although it looks worse in
laboratory B117 cabinet corrosion tests, it usually
performs much better than hard chrome in service.
HVOF Spray gun.
HVOF coating of a landing gear hydraulic
cylinder (courtesy Southwest United Industries). |
